Disbanding was absolutely the right thing to do. The internal loyalties of any army could be hidden for years until a revolution day when simple nostalgia would motivate an unstoppable old-school revolt.
The real reason Bremer disbanded the army was we did not have an official end to hostilities when Baghdad fell. With Saddam, his sons, and most of the leadership running loose with a few hundred million in cash it would have been foolhardy to keep 400,000 armed men running around. It would have been all too easy to bribe a division or corp to launch a coup or start the war all over again. Don't forget how Napoleon was able to escape Elba, gather his old corp commanders back (they had sworn loyalty to the king) and restart the war that would end at Waterloo.